The present invention relates in general to synchronous detection of AC signals, and more particularly concerns novel apparatus and techniques for improved rejection of noise by means of synchronous switching.
Synchronous detection has been employed for a long time to detect small AC signals buried in noise. One common application of synchronous detection is the lock-in amplifier in which the signal is multiplied with a square wave generated from a reference signal. The result is then applied to a low-pass filter. In this way a DC voltage is generated that is proportional to the amplitude of that part of the detected signal that coincides in phase (and frequency) with the reference signal. Spurious signals of frequencies different from the reference frequency (.nu..sub.ref) (or its odd harmonics) when mixed with the reference signal give rise to beats that are suppressed in the low-pass filter normally used according to prior art.
The time constant of the low-pass filter has to be chosen very long if the disturbing signals are large compared with the wanted signal. This leads to the inconvenience that strong short disturbances, such as transients from power switches or starting electrical motors, block the lock-in amplifier for a long time after the disturbance has ceased to exist. Pick-up from the main supply is one common source of noise. 50 (or 60) Hz rumble is then superimposed on the signal and, in spite of careful shielding, the pick-up noise very commonly dominates at signal levels below 1 .mu.V.
A summary of the subject may be found in two articles by Charles A. Nittrouer in the September and October 1968 issues of the Electronic Instrument Digest, and these articles are hereby included by reference in the present disclosure.
The development of new integrated circuits has led to attempts to improve the rejection of noise in synchronous detection which can be reached by means of RC filter or the like. An example of this is described in an article by Komachi and Tanaka entitled "Lock-in Amplifier using a sampled-data synchronous filter" and published in J. Sci. Instr. 8 967 (1975). The idea presented is to use a number of capacitors and a differential amplifier which receives on one input the voltage across a capacitor which is being charged during a given time interval, and on the second input the voltage from a capacitor that has been charged during an earlier time interval. Komachi and Tanaka have demonstrated how this arrangement can be used as a narrow band-pass filter. There are however disadvantages with their design as, for example, the fact that the input signal, via an LP filter, is always connected to the input of the differential amplifier and hence will affect the output instantaneously. If the RC time of the filter is very long, a disturbance that has occurred during a certain time period will affect the output for a long time.